This invention relates generally to radio frequency antennas, and more particularly to radio frequency array antennas.
As is known in the art, it is sometimes desirable to arrange a plurality of antenna elements in an array to provide an antenna with a relatively narrow beam width. As is further known in the art, in an array antenna the spacing "a" between the centers of adjacent antenna elements must be less than [1- (1/N)].lambda..sub.H /(1+sin .theta.)] (where N is the number of antenna elements along a scan axis of the array antenna, .lambda..sub.H is the wavelength of the highest operating frequency of the array antenna and .theta. is the maximum angular duration of the beam from the boresight axis of the array antenna) in order to obtain satisfactory grating lobe reductions. Thus, in wide bandwidth, wide-scan angle arrays, the element to element spacing, "a", becomes very small in terms of wavelengths at the lower operating frequencies in order to avoid grating lobes at the higher operating frequencies. As is also known in the art, one type of antenna element used in an array antenna is a horn. When such horn antenna elements are disposed in an array, and more particularly stacked one aside another in a direction along the E-field of the propagating energy, there is a degree of electrical coupling among the elements, particularly when the antenna is operating at the lower frequency region of the operating bandwidth, with the result that the gain of the antenna is degraded at the lower operating frequencies of the antenna.